1. Field of the Invention
The present invention relates to an illuminant adjustment technique, and more particularly, to an LED illuminant driving circuit and an automatic brightness compensation method thereof.
2. Description of Related Art
FIG. 1 shows a block diagram of a conventional LED illuminant device. Referring to FIG. 1, an LED illuminant device 10 includes an electronic transformer 100, a rectifier circuit 110, an input capacitor Cin, a driving circuit 120, and an LED illuminant 130. First, an output of an electronic transformer 100 with a conventional alternating current (AC) to AC transformation is a high frequency AC voltage VAC2. For example, when an AC voltage VAC1 with low frequency of 60 Hz is applied, the AC voltage VAC1 is transformed by the electronic transformer 100 to an AC voltage VAC2 with high frequency of 25 KHz to 100 KHz, wherein root-mean-squared voltage (Vrms) of the AC voltage VAC1 and the AC voltage VAC2 is 110V and 12V respectively. Then, the AC voltage VAC2 output from the electronic transformer 100 is connected with the rectifier circuit 110 and the input capacitor Cin. The rectifier circuit 110 and the input capacitor Cin produces a direct current (DC) voltage VDC with ripple components on the AC voltage VAC2. Furthermore, a power input terminal VIN of the driving circuit 120 receives a DC voltage VDC to make the LED illuminant 130 emit light.
Referring to FIG. 2, FIG. 2 shows the DC voltage VDC with ripple components of FIG. 1. When the driving circuit 120 utilizes the DC voltage VDC with ripple components as an input power source, the following conditions may occur:
(1) The ripples of the DC voltage VDC causes a periodic operation of a starting time and a stopping time so as to impact on a working stability of the driving circuit 120.
(2) The ripples of the DC voltage VDC influences the LED illuminant 130 at an output terminal of the driving circuit 120, and the ripple which causes variations of average current or average voltage of the LED illuminant 130 also causes the variations of brightness of the LED illuminant 130
In the driving circuit 120 of the related art, due to the impact from the DC voltage VDC with ripples, the average current or the average voltage conducted through the LED illuminant 130 will not be controlled stably. It also means the brightness of the LED emitting light may be different so it causes inconvenience of applications. Therefore, there is a need for an LED illuminant driving circuit and an automatic brightness compensation method thereof accordingly.